1. Field of the Invention
The present invention relates to instrument technology and has specific reference to apparatus for preparing gas mixtures from constituents taken in a given proportion.
The invention will find application in gas analysing engineering for the preparation of calibrating gas mixtures and as a synthesizer of the gas mixtures used in life-support systems, medical and biological research, in epitaxial-growth and evaporation processes.
2. Description of the Prior Art
Calibration gas mixtures are in wide-spread use, being supplied in high-pressure cylinders (up to 14 Mpa) to customers. They are prepared by introducing the constituents one after another, whereby the pressure of each succeeding component must be higher than that of the preceding one. The composition of the gas mixtures produced in this way lacks reliability and must be tested with the aid of reference gas analysers before using such mixtures for calibration purposes. The existing practice of preparing gas mixtures in high-pressure cylinders calls for setting up a complicated system of storing, shipping and servicing the cylinders in vast numbers at high cost. Not excluded is a deterioration of the composition of the gas mixture in this case or its ineffective utilization. Precisely reproducing a given composition of a mixture is a problem.
As a result the recourse is made to continuous dynamic installations capable of producing gas mixtures of given composition under low pressure in situ.
Widely known is an apparatus for the preparation of a gas mixture from constituents used in a given proportion which is employed mainly in equipment for gas analysis of blood (DE; C; 2,123, 691). It has two flow paths each of which incorporates a source of gas mixture constituent, a pressure-reducing valve, two filters and a gas pressure controller which are all connected in series and linked to a pair of orifice plates connected in parallel at the downstream sides. The outflow from an orifice plate in the first flow path is connected to the outflow from an orifice plate in the second flow path and the outflow from the other orifice plate in the first flow path is connected to the outflow from the other orifice plate in the second flow path. The orifice plates are contained in a constant-temperature container, and the flow areas of the orifices have a ratio of 17:17:2:1. The outflow of each gas mixture constituent from its source enters the pressure-reducing valve for reducing the pressure to a specified level before reaching the two filters, designed to separate moisture and sediments, and the gas pressure controller which accurately maintains the gas pressure at a specified value. The outflow from the pressure controllers is passed through the two orifice plates in each flow path so that finally there are produced two streams of gas mixture the proportion between the constituents of which is determined by the diameters of the flow areas of the orifices.
The known apparatus is incapable of producing gas mixtures of precisely specified composition. The proportion between the gas mixture constituents is a function of the flow rates of these constituents through the orifices which cannot be accurately determined due to lack of accurate flowmeters, inaccurate formulae for calculating the flow rates through the orifices and the effect of non-informative variables (temperature, barometric pressure) which must be taken into account during these calculations. Therefore, the known apparatus must be tested for performance not with the aid of gas flowmeters but using reference gas analysers. An unavoidable difference between the pressure at the upstream sides of the orifice plates and the barometric pressure influences the proportioning of the gas mixture constituents so that the known apparatus cannot guarantee an exact reproduction of a given composition of the gas mixture.
Also well-known is an apparatus for producing a gas mixture from constituents taken in a given proportion which is used in calibrating gas analysers (US; A; 3,856,033). The known apparatus features several feed lines each consisting of series-connected sources of gas mixture constituents, pressure-reducing valves, stop valves, a regulating valve connected to the outlets from all stop valves, a gas mixer the inlets of which are connected to the regulating valve and to the sources of gas mixture constituents, a compressor the inlet of which is connected to the gas mixer, a gas analyser the inlet of which is connected to the high-pressure feed line downstream of the compressor and a means of sensing deviations in the value of the proportion between the gas mixture constituents from a set value which serves to control the operation of the regulating valve.
A flow of every gas mixture constituent enters the gas mixer, and the composition of the mixture formed there is monitored by the gas analyser. Any deviation from a given composition is converted into an analogue signal which controls the operation of the regulating valve. The proportioning of the gas mixture constintuents is under a continuous control in the apparatus and is effected by changing the flow rates of the constituents. The apparatus thus employs the feed-back principle but cannot ensure good accuracy of proportioning the gas mixture constituents owing to errors which occur during the operation of automatic gas analysers, in converting gas analyser signals and controlling the flow rates of the constituents.
Further known is an apparatus for controlling the composition of gas mixtutes (SU; A; 643,848) incorporating sources of gas mixture constituents to the outlets of which there are connected pressure-reducing valves and a means of setting a proportion between the gas mixture constituents which consists of a distributor and two sets of capillary tubes series-connected to the distributor. The number of the capillary tubes is decided by the number of the gas mixture constituents, and the friction heads of the tubes relate as 1:2:2:5. The known apparatus is provided with a mixer of the gas mixture constituents in the form of a fluidic element with three series-connected loop chambers. The midmost thereof is filled with a nozzle connected to a pressure-reducing valve, and facing the nozzle is a confuser of an ejector with a mixing passage connected to a header the outlet of which is the outlet from the apparatus. The inlets into the extreme loop chambers of the fluidic element are linked to the sources of gas mixture constituents by way of the capillary tubes and the distributor.
In the known apparatus the proportioning of the gas mixture constituents is controlled by their flow rates through the capillary tubes and ejector. But since these flow rates can be neither calculated nor directly measured with a good degree of precision, the proportioning of the constituents also lacks accuracy. Instability of ejector performance and the effect of noniformative variables (barometric pressure, temperature, work load) cause time-dependent variations of the flow rate values, both absolute and relative to each other. No accurate determination of the relation between the friction heads of the capillary tubes is possible due to a non-linear character of the value, lack of adequate equipment and metrological difficulties.